Some stains caused by beverages, food, or oily substances may not be visible after they dry. But later, with exposure to heat or simply the passage of time, a yellow or brownish stain will appear. This is the end result of oxidation or caramelization of sugar or sweetening agents. It is the same process that makes a peeled apple turn brown after exposure to air. If we don’t know about it we can’t fix it, so let us know if you spilled something.